Cemented carbide is a general name for an alloy prepared by powder metallurgy from 9 types of metal carbides from groups IVa, Va and VIa in the periodic table of iron group elements and metals, such as Fe, Co and Ni.
The carbide phase makes the alloy have high hardness and wear resistance, while the bonding phase gives the alloy certain strength and toughness.
According to the composition, cemented carbides can be divided into five categories : tungsten carbide-based cemented carbides, titanium carbide-based cemented carbides, coated cemented carbides, steel-based cemented carbides and other cemented carbides.
According to its scope of application, carbide can be divided into four categories : carbide cutting tools, carbide molds, carbide measuring tools and wear-resistant parts and carbide for mining and petroleum geology .
Generally speaking, WC Co cemented carbides are widely used, such as cutting tools, metal drawing dies, stamping dies, measuring tools for cast iron, non-ferrous metals and their alloys, and wear-resistant parts for mining machines and geological exploration;
WC Ti Co alloys are mainly used for cutting steel;
WC TiC – (NbC) – Co alloys are mainly used to cut parts made from high hardness materials.
Although other types of cemented carbides have made great progress in recent years and achieved great success in some special applications, WC Co series cemented carbides (i.e. YG type) have excellent comprehensive mechanical properties, which are the most widely used cemented carbides and used in industry.
1. Problems encountered in brazing carbide
The brazing of carbide is low.
This is because the carbon content of carbide is high and the dirty surface generally contains more free carbon, which makes weld wetting difficult.
In addition, carbide is easy to oxidize and form an oxide film at brazing temperature, which will also affect the wettability of the weld.
Therefore, cleaning the surface before brazing is very important to improve the wettability of brazing filler metal on carbide.
If necessary, measures such as copper or nickel plating can be taken on the surface.
Another problem in carbide brazing is that the joint is easy to break.
This is because its coefficient of linear expansion is only half that of low-carbon steel.
When carbide is brazed with the matrix of this type of steel, it will generate a large thermal stress in the joint, which will lead to cracking of the joint.
Therefore, crack prevention measures must be taken when brazing carbide with different materials.
2. Surface treatment before brazing
Before brazing, the oxides, grease, dirt and paint on the surface of the part must be carefully removed, as the molten solder cannot wet the surface of the parts that have not been cleaned, nor fill the joint gap.
Sometimes, to improve the brazing of the base metal and the corrosion resistance of the welded joint, parts must be pre-coated with a certain layer of metal before brazing.
(1) Oil stain can be removed with organic solvent
Common organic solvents include alcohol, carbon tetrachloride, gasoline, trichlorethylene, dichloroethane, and trichloroethane.
During small batch production, the scratch can be cleaned by immersion in organic solvent.
The most commonly used in mass production is organic solvent vapor degreasing.
Furthermore, satisfactory results can be obtained in hot alkaline solution.
For example, steel parts can be degreased by soaking in 10% caustic soda solution at 70-80℃, and copper and copper alloy parts can be cleaned in a solution of 50g trisodium phosphate, 50g sodium bicarbonate and 1L of water at a temperature of 60 -80°C.
The parts can also be degreased with detergent and then cleaned carefully with water.
When the surface of the part can be completely wetted by water, it indicates that the surface grease has been removed.
For small parts with complex shapes and large quantities, ultrasonic cleaning can also be used in special grooves.
The efficiency of ultrasonic oil removal is high.
(2) Oxide removal
Before brazing, the oxides on the surface of the workpiece can be processed by mechanical method, chemical etching method and electrochemical etching method.
File, metal brush, sandpaper, grinding wheel and sandblasting can be used to remove the oxide film from the surface when cleaning by mechanical method.
Cleaning with a file and sandpaper is used to produce a single part, and the groove formed during cleaning also favors the wetting and spreading of the weld.
Grinding wheel, metal brush, sand blasting and other methods are used in batch production.
Mechanical cleaning is not suitable for aluminum, aluminum alloy and titanium alloy surfaces.
(3) Base metal surface coated with metal
The main purpose of metal coating on the surface of the base metal is to improve the weldability of some materials and increase the wettability of the weld to the base metal;
Prevent the interaction between the base metal and the filler metal from negatively affecting the quality of the joint, such as preventing cracks and reducing brittle intermetallic compounds at the interface;
As a solder layer, it simplifies the assembly process and improves productivity.
3. Brazing materials
(1) Filler metal
Brazing tool steels and hard alloys generally use filler metals of pure copper, copper, zinc and silver copper.
Pure copper has good wettability for all types of cemented carbides, but the best effect can be obtained by brazing in a hydrogen reducing atmosphere.
At the same time, due to the high brazing temperature, the stress on the joint is large, leading to an increased tendency to crack.
The shear strength of the joint with traditional pure copper brazing is about 150MPa, and the plasticity of the joint is also high, but it is not suitable for high temperature work.
Copper-zinc filler metal is the most commonly used filler metal for brazing tool steels and cemented carbides.
In order to improve the wettability of the filler metal and the strength of the joint, Mn, Ni, Fe and other alloying elements are often added to the filler metal.
For example, 4%w(Mn) is added to B-Cu58ZnMn to make the shear strength of carbide welded joints reach 300~320MPa at room temperature and maintain 220~240MPa at 320°C.
By adding a small amount of Co based on B-Cu58ZnMn, the shear strength of the welded joint can reach 350MPa, and it has higher impact resistance and fatigue resistance, which significantly improves the service life of drilling tools and tools. rock.
The melting point of the silver and copper filler metal is low and the thermal stress generated by the brazed joint is small, which helps to reduce the tendency of the carbide to crack during brazing.
In order to improve the wettability of the filler metal and improve the strength and working temperature of the joint, Mn, Ni and other alloying elements are often added to the filler metal.
For example, the filler metal B-Ag50CuZnCdNi has excellent wettability to carbide, and the welded joint has good comprehensive properties.
In addition to the above three types of solders, Mn and Ni based solders such as B-Mn50NiCuCrCo and B-Ni75CrSiB can be used for cemented carbides that operate above 500°C and require high joint strength.
For brazing high-speed steel, special brazing filler metals that match the brazing temperature and quenching temperature must be selected.
This type of brazing filler metals can be divided into two categories.
One of them is ferromanganese-type brazing filler metals, which are mainly composed of ferromanganese and borax.
The shear strength of welded joints is generally about 100 MPa, but the joints are prone to cracking.
The other is special copper alloys containing Ni, Fe, Mn and Si.
Joints welded with it are not prone to cracking and the shear strength can be increased to 300MPa.
(2) Flow and shielding gas
Brazing flux selection must match the base metal to be welded and the brazing filler metal selected.
Related Reading: Welding Flux: How to Select and Use Correctly
In brazing tool steel and carbide, the flux used is mainly borax and boric acid, and some fluorine (KF, NaF, CaF2, etc.) is added.
Copper and zinc solders are equipped with FB301, FB302 and FB105 fluxes, and silver copper solders are equipped with FB101～FB104 fluxes.
When special brazing filler metal is used for brazing high-speed steel, borax brazing flux is mainly used.
To avoid oxidation of tool steel during brazing heating and to avoid cleaning after brazing, gas shielded brazing can be used.
The shielding gas can be inert gas or reducing gas, and the dew point of the gas must be lower than –40 ℃.
Carbide can be brazed under the protection of hydrogen, and the required hydrogen dew point should be lower than -59 ℃.
4. Brazing process
Tool steel must be clean before brazing and the machined surface does not need to be very smooth to facilitate wetting and spreading of materials and fluxes.
The surface of the carbide must be sand blasted before brazing, or polished with silicon carbide or diamond grinding wheel to remove excess carbon on the surface, so as to be moistened by the filler metal during brazing.
Cemented carbide containing titanium carbide is difficult to be wetted, so the wettability of brazing is increased by coating its surface with copper oxide or nickel oxide paste in a new way and cooking in a reducing atmosphere to make the copper transition or nickel to the surface.
Brazing of carbon tool steels should preferably be carried out before or at the same time as the quenching process.
If brazing is carried out before the quenching process, the solidus temperature of the filler metal used must be higher than the quenching temperature range, so that the solder can still have sufficient strength when reheated to the quenching temperature without failure.
When brazing and quenching are combined, filler metal with a solidus temperature close to the quenching temperature should be selected.
The composition range of tool steel alloy is very wide.
The suitable brazing filler metal, heat treatment process and combination technology of brazing and heat treatment processes should be determined according to the specific type of steel so as to achieve good joint performance.
The quenching temperature of high-speed steel is generally higher than the melting temperature of silver, copper and copper-zinc solders, therefore it is necessary to perform quenching before brazing and during or after secondary tempering.
If quenching is required after brazing, only the above-mentioned special filler metal can be used for brazing.
When brazing high-speed steel tools, it is appropriate to use a coke oven.
When the filler metal melts, take out the tool and pressurize it immediately, extrude the excess filler metal, conduct quenching in oil, and then quench at 550~570℃.
When brazing the carbide tool with the steel tool shank, it is advisable to increase the brazing seam gap and apply a plastic compensating gasket to the brazing seam and cool slowly after welding to reduce the brazing stress, avoid cracking and prolong the service life of the carbide tool assembly.
5. Cleaning after brazing
Most of the flux residue corrodes the welded joint and also makes the welded joint difficult to inspect, so it needs to be cleaned.
The flux residue in welding should be washed with hot water or general slag removal mixture, and then pickled with appropriate pickling solution to remove the oxide film on the base tool.
However, do not use nitric acid solution to avoid corrosion of the brazing metal.
Organic solder flux residue can be cleaned or cleaned with gasoline, alcohol, acetone and other organic solvents;
Zinc oxide and ammonium chloride residues are highly corrosive, so they must be cleaned in 10% NaOH solution and then cleaned with hot or cold water.
Borax and boric flux residues are usually resolved by mechanical methods or prolonged immersion in boiling water.
6. Brazing quality inspection
Welded joint inspection methods can be divided into non-destructive inspection and destructive inspection.
The following are mainly non-destructive testing methods:
(1) Visual inspection.
(2) Dye test and fluorescence test.
These two methods are mainly used to check defects such as microcracks, air holes and gaps that cannot be found by appearance inspection.
